Serum 25-hydroxyvitamin D 3 and risk of type 2 diabetes among Japanese adults: the Hitachi Health Study.

Background & Aims: Increasing evidence has suggested a protective role of vitamin D on diabetes, but epidemiologic evidence is scarce among Asian populations. Here we prospectively examined the association between serum 25-hydroxyvitamin D ₃ (25[OH]D ₃ ) and type 2 diabetes (T2D) risk in Japanese individuals.
Methods: A nested case-control study was conducted in a biomarker cohort of 4754 employees (baseline age 34-69 years) who had attended in a comprehensive health checkup and donated a blood sample. Diabetes diagnosis was based on plasma glucose, glycated hemoglobin, and self-reporting during the 5-year follow-up. Using density sampling, two controls were randomly matched to each case by sex, age, and date of checkup; 336 cases and 668 controls had serum 25(OH)D ₃ data. Association between serum 25[OH]D ₃ and the risk of T2D was assessed using conditional logistic regression analysis.
Results: Serum 25(OH)D ₃ was significantly and inversely associated with T2D risk after adjustment for known risk factors other than BMI (OR [highest vs. lowest serum 25(OH)D ₃ quartile] = 0.58, 95% CI = 0.36-0.92; P for trend = 0.03). This association was somewhat attenuated after additional adjustment for BMI (OR = 0.65, 95% CI = 0.40-1.08; P for trend = 0.08). The inverse association was more evident among individuals whose blood samples were taken during the darker season (OR = 0.45; P for trend = 0.01). In the highest quartile of 25(OH)D ₃ , progression from prediabetes to T2D was about 37% lower than in the lowest quartile.
Conclusions: Higher circulating 25(OH)D ₃ was associated with a lower risk of T2D, and this association was stronger among individuals whose blood was taken during the darker season and among those with prediabetes.
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